Recently, great progress has been made in the study of the intracellular machinery that drives the cell division cycle. The family of cyclin- dependent kinases (cdks), in conjunction with their regulatory partners, the cyclins, have been shown to play key roles in proliferation. Further, expression of protein inhibitors of cdk activity (such as p21, p27, or members of the INK4 family) has been shown to cause cell cycle arrest and to promote the timely and orderly transition from proliferation to differentiation. Despite this rapidly expanding body of information, astonishingly little is known about the regulation of proliferation in developing mammalian central nervous system. We wish to dissect the cell cycle regulatory mechanisms that promote the orderly transition from proliferation to differentiation to yield the correct number of neurons during neuronogenesis. Data derived from our experiments in the p27-/- knockout mouse strongly support such a regulatory role for this kinase inhibitory protein during the proliferative phase of mammalian central nervous system development. This view is supported by the phenotype which we have observed in mice harboring a homozygous deletion of the p27 kip1 gene. Although these mice are morphologically normal and fully viable, they are 40% larger and have multi-organ hyperplasia. on average, the brain of p27-/- animals exhibits 30% greater neuronal density and 20% larger brain size than is found in wildtype indicating that the knockouts harbor a much higher number of neurons in brain. Since p27 is a bona fide cdk inhibitor, the greater number of neurons present is most likely a consequence of hyper- proliferative activity of neuronal precursors during neuronogenesis due to a loss of the p27 protein. We will analyze the developmental profile of precursor proliferation and the output of post-mitotic neurons during corticogenesis in these animals. The responsiveness of neuronal precursors to known polypeptide growth factors will also be tested. The p27-/- mice will serve as a critical model system for us to dissect the molecular pathway that regulates the histogenesis of the nervous system. The p27-/- animals also develop pituitary tumors by the age of 10 weeks with 100% penetrance. The properties of the tumors are indistinguishable from the pituitary tumors developed by Rb knockout chimeric mice. most likely, a loss of p27 and pRb affects the same cellular pathway that leads to tumorigenesis in these cells. Using the p27-/- animal model, we will investigate the regulatory mechanisms involving both these gene products to elucidate the regulation of proliferation in this central nervous system tissue, as well.